Bar guns assemblies are used to selectively receive a multiplicity of different flavored syrups from a multiplicity of pressurized sources and to mix the syrup with soda water and dispense the resulting beverage into a container. Buttons on the bar gun actuate pistons in piston chambers which, when unseated, will allow fluid to flow through the chambers to the nozzle for mixing and dispensing. Undesirable leakage has occurred from the piston chambers.